Battery Capacity and Runtime Calculator

This battery capacity calculator works out how long a battery will run and how much energy it holds, using nothing more than the numbers printed on the battery and the load you plan to put on it. Enter the battery's capacity in amp-hours, the current draw of your load in amps, and the battery's voltage, and the calculator instantly returns the estimated runtime in hours, the stored energy in watt-hours, and the load current you entered for reference. The maths is straightforward: runtime is capacity divided by current, and energy is capacity multiplied by voltage. It is useful whenever you need a quick sizing check, choosing a battery for a solar setup, a camping fridge, a drone, a UPS, or any device where you know the load but not how long the battery will last. Change any of the three inputs and the results update immediately, so you can compare different battery sizes or work backwards from a required runtime to the capacity you need to buy. Keep in mind that real batteries rarely deliver their full rated capacity once you push them hard: high discharge currents, cold temperatures and normal ageing all shrink the usable amp-hours, an effect engineers call the Peukert effect. Treat the figures here as a best-case starting point and build in a safety margin, particularly for anything safety-critical or hard to recharge in the field.

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Capacity (Ah)

Load current (A)

Voltage (V)

Energy (Wh)

Load current

The formula

Runtime in hours is the capacity in amp-hours divided by the load current in amps: t = Ah / A. The stored energy is watt-hours = Ah times the voltage. Real capacity falls at high discharge rates (Peukert effect).

Worked example

A 100 Ah battery supplying a 5 A load runs for 100 / 5 = 20 hours, and at 12 V stores 100 times 12 = 1200 watt-hours. Enter 100, 5, 12 to confirm.

Frequently asked questions

How do I work out battery runtime?

Divide the capacity in amp-hours by the load current in amps. A 100 Ah battery at 5 A lasts about 20 hours.

How much energy does a battery store?

Multiply the amp-hours by the voltage to get watt-hours.

Why is real runtime shorter?

High discharge currents, temperature and ageing reduce usable capacity, an effect captured by Peukert’s law. Allow a margin.

Who this calculator is for

This calculator is for electronics students, hobbyists and engineers.

What this calculator assumes

You enter values in the units shown.
Ideal components are assumed.
Results are rounded for display.

Formula and sources

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